To improve a glare reducing property and a heat shielding property, a laminated glass for vehicles such as a windshield sometimes has a band-shaped shade region (a shade band) that is colored green, blue or the like. Although a shade-band may be provided on a surface of a glass sheet, it generally is formed by partially coloring an interlayer for joining glass sheets along a strip. There is a region in windshields where by law a visible light transmittance should be a predetermined value (e.g. 70%) or more. In this description, this field hereinafter is often referred to as “a visual field region”. Thus, a shade-band should be formed out of the visual field region, and generally is arranged in an upper area of the windshield.
To absorb heat radiation and ultraviolet rays, colored glass sheets frequently are used for a laminated glass for vehicles, especially for automobiles. Typical colored glass sheets include a green glass sheet that contains more iron-based ingredients than usual. Nowadays, it is proposed that an interlayer also has an infrared ray absorbing function. For example, this function can be added to the interlayer by dispersing electrically conductive minute particles of ITO (indium tin oxide) or the like into the film made of a thermoplastic resin such as polyvinylbutyral (PVB), as disclosed in JP8-259179 A. This infrared ray absorbing function derived from an interlayer mostly is imparted to the whole area of the windshield that includes the visual field region.
In recent years, the need for installing various devices in vehicles has arisen so as to achieve an ITS (intelligent transport system) communication or the like. Examples of the devices include a photoreceiver such as a CCD (charge coupled device) camera or a rain sensor for detecting the light that has a wavelength in a visible and infrared range and that passes through a window glass, especially a windshield. When the photoreceiver is installed on the interior surface of a windshield, it is desirable that the photoreceiver is arranged in an upper region of the windshield so as to secure safety and exert the function of the photoreceiver sufficiently.
However, as explained above, a shade-band frequently is formed in the upper region. The shade band considerably lowers the light transmittance of the windshield in a visible and infrared range, and thus deteriorates the sensitivity of the photoreceiver that receives the light through the windshield. Therefore, conventionally, the position of the photoreceiver is forced to change or the use of the photoreceiver with the shade band should be abandoned. Similarly, when the windshield includes an interlayer that has a heat shielding property, the sensitivity of the photoreceiver cannot help decreasing, especially when the photoreceiver detects infrared rays.
Some vehicles have a HUD (head-up display) for displaying information that is necessary for driving such as a vehicle speed on a windshield. A HUD requires a light source installed inside the vehicle and a reflectance increasing film (combiner) for reflecting the light from the source toward the eyes of the operator. In general, the combiner is formed on an inside surface of the windshield within its visual field region.
However, in the windshield that includes a heat-shielding interlayer, the film as well as a combiner lowers the visible light transmittance of the windshield. Thus, the transmittance in the region where the combiner is formed is sometimes below the legal limit. When the windshield includes a colored glass sheet that is excellent in a heat shielding property, the replacement of the colored glass sheet by a normal clear glass sheet increases the visible light transmittance of the windshield. However, this replacement for increasing the visible light transmittance in a partial region decreases the heat shielding property over the whole region.
As explained above, (1) a shade band and a photoreceiver, (2) a heat shielding interlayer and a photoreceiver, and (3) a heat shielding interlayer and a combiner are difficult to be made compatible with each other in a practical use since the shade band and the heat shielding film decrease the light transmittance.
Conventionally, when the need for partially changing the optical property of the windshield arises, a thin film is partially formed on the glass sheet. This is convenient to specify the region where the property should be changed for a HUD combiner or the like since the glass sheet is cut into a predetermined shape prior to a lamination step. A ceramic print that should be formed in a frame shape in a windshield is printed on the surface of the glass sheet.
On the other hand, when the need for imparting some function to a relatively wide region or the whole region of a windshield arises, the function is frequently added into an interlayer of the windshield. This is because the addition of functional materials into an interlayer that is made of organic materials is often advantageous in reducing a manufacturing cost, and an interlayer is used for forming a shade band or adding electrically conductive minute particles, for example. In most cases, interlayers are supplied to a lamination step in a rolled-up state, and thus it is considered inappropriate to treat them partially in regions that correspond with predetermined regions of windshields.
Thus, under these circumstances, to achieve the combinations of (1) to (3), an anti-reflecting film should be formed on a predetermined region of a windshield where a photoreceiver should be arranged so as to increase the light transmittance of the region. However, when relying on such a conventional approach, the sensitivity of the photoreceiver cannot be restored sufficiently or the reflectance of the combiner needs to be lowered for the legal limitation on the visible light transmittance.